Andrew: Glory to God! Many years.
Do Orthodox faithful reject the concept of development?
In the sense that Roman Catholics use the term, yes, Orthodox reject the idea of development of doctrine.
The classic expression of what Orthodox understand as valid 'development' is this passage from St. Vincent of Lerins:http://www.voskrese.info/spl/lerins23.html
In essence, we believe that the fundamental dogma was 'delivered once for all' to the Apostles. Our understanding of that Apostolic doctrine can be expanded, its implications explored, but the dogma itself cannot be changed nor can things which were not part of the original revelation of Christ Jesus be made part of that binding dogma.
To put it another way, our understanding of development of dogma is that if one took the various dogmatic definitions of the Ecumenical Councils and went back in time to present them to St. Peter, his first response might be, 'wait a second, I'm a simple fisherman who only learned Greek later in life--what do these technical terms 'homoousious', 'hypostatic union,' etc mean?' But if the terms were explained, then St. Peter's response would be 'Yes, you may be using philosophical Greek, or Russian or English, but the underlying concepts, yes, that's exactly what was revealed to me by the Holy Spirit and in person by the Risen Lord and what I taught."
If something is *not* part of that fundamental deposit, if St. Peter or St. Paul wouldn't be able to say, "Yes, I may have used different terms, but that's what I taught." then it is not dogma or doctrine. It may be beneficial (as the Fathers of Nicea determined celebrating Pascha on one day through the Church would be, or in the specific fasting rules that the Church has developed over time) but it is not 'necessary to salvation' and its certainly cannot trump the original revelation.
Do Orthodox believe in a visible unity in the Church? If so, how is it maintained?
Yes we believe in visible unity. Our unity is in the Cup of Communion. Each local parish is unified to its bishop through participation in the mystery at which the bishop is commemorated. In turn, when the bishop participates in the mystery, he commemorates the presiding hierarch of his synod, so that the synod/local church is visibly one. And when the presiding hierarch participates in the mysteries, he commemorates the presiding hierarchs of all the other local churchs, who commemorate him in turn and thus the Church is unified throughout the world in the myster of Communion.